1. Field of the Invention
The present invention relates to an image forming apparatus based on electrophotography for obtaining an output image by transferring a toner image held on an image carrier onto a transfer material, and particularly to an image forming apparatus which prevents transfer memory from taking place due to different sizes of the transfer materials.
2. Description of the Related Art
In an image forming apparatus of the prior art, a transfer material carrier holding a transfer material thereon is moved to a transferring position where a toner image held by an image carrier is transferred onto the transfer material thereby to obtain an output image. For this type of image forming apparatus, such a configuration has been known as a belt or a cylindrical drum is used as the transfer material carrier and a transfer charger of contact type or non-contact type is arranged on the inner surface of the belt or the cylindrical drum for transferring the toner image held by the image carrier onto the transfer material.
An example of such an image forming apparatus of belt transfer type utilizing a transfer belt is described in the Journal of Electrophotography Association vol. 33, No. 1 (1994). Constitution of this image forming apparatus will be described below with reference to FIG. 18. The image forming apparatus has, around a photosensitive drum 101 which has an electrostatic latent image formed thereon by scanning exposure and is located at the center, an electrostatic charger 102 for charging the surface of the photosensitive drum 101 with electrostatic charges, a developer 104 which develops the electrostatic latent image by means of toner, an image transfer corona discharger 116 and a transfer belt 105 for transferring the toner image formed through development onto copy paper P, a cleaner 106 which cleans the photosensitive drum 101 upon completion of one imaging cycle, an eraser 107 which removes the electrostatic charge from the surface of the photosensitive drum 101, and other components.
The transfer belt 105, which comes into proximity to the photosensitive drum 101 at a transferring position and is driven to run in synchronization with the rotation of the photosensitive drum 101, is stretched across three metal rollers, namely a drive roller 118, a driven roller 119 and a pre-transfer roller 121. The pre-transfer roller 121, which makes contact with the transfer belt 105 immediately before the transferring position, is grounded thereby to allow the electrostatic charge to collectively flow into the earth and prevent the potential from building up, for the purpose of preventing `premature scatter` from taking place which is such a phenomenon as the toner moves through a narrow space between the photosensitive drum 101 and the transfer belt 105. The pre-transfer roller 121 has another purpose of assisting in the discharge of the image transfer corona discharger 116. Installed around the transfer belt 105 is a belt cleaner 120 which cleans the outer surface of the transfer belt 105 by means of a blade making contact with the transfer belt 105. The pre-transfer roller 121 may be omitted.
In the image forming apparatus constituted as described above, an output image is obtained through such operations as described below. First, copy paper P (transfer material) fed out by a paper feed roller from a paper tray is, after being synchronized with the photosensitive drum 101 by the timing roller 109, sent onto the transfer belt 105 and is then sent to a transfer section by the transfer belt 105. At this time, because the transfer belt 105 is supplied with electric charge by the image transfer corona discharger 116, the copy paper P experiences electrostatic polarization due to an external electric field generated by the charge. When the copy paper P reaches the transferring position between the photosensitive drum 101 and the transfer belt 105, the toner is transferred onto the copy paper P. As the pre-transfer roller 121 is brought into contact with the transfer belt 105 immediately before the transferring position, `premature scatter` of the toner held on the photosensitive drum 101 moving through a narrow space between the photosensitive drum 101 and the transfer belt 105 is prevented from taking place, thereby ensuring reliable transfer of the toner onto the copy paper P. Then the copy paper P is moved by the transfer belt 105 to a fixing section, where the toner is fixed by a fixing roller 108 thereby to obtain the output image.